In plasma processing, the shrinking feature sizes and the implementation of new materials in the next generation of device fabrication have put new requirements on plasma processing equipment. The smaller device features, larger substrate size, and new processing techniques (involving multi-step recipes, such as for dual-damascene etch) have increased the challenge to maintain good uniformity across the wafer for better device yields.
In capacitively coupled RF plasma reactors, the electrode opposite to the substrate electrode is generally called the upper electrode. The upper electrode could be grounded, or have one or more radio frequency (RF) power sources attached to it. The substrate electrode is generally called the lower electrode. A mechanical arrangement for a lower electrode in a capacitively coupled plasma processing chamber may involve cantilevering the assembly that includes the lower electrode from a side of the chamber. This cantilevered lower electrode can be a fixed distance from the upper electrode or can be designed for a variable distance from the upper electrode. In either case, parallelism of one electrode surface to the other is generally a critical mechanical parameter that can affect the process performance on the wafer.
Due to added complexity, many capacitively coupled RF plasma reactors forgo the feature of precise parallelism adjustment between electrodes and rely on tight manufacturing tolerances of the assembly components to keep parallelism within acceptable limits. This approach typically adds cost to those components and may limit the ultimate parallelism specification that can be achieved. Other arrangements include slots or clearance holes in mating parts allowing free play to adjust parallelism during assembly. This approach is time consuming and usually requires repetitive processes to achieve the correct configuration. The approach may also require the plasma processing system to be disassembled to some extent to adjust the necessary components. Other methods attempt to provide a means for adjustment, but may have no direct means to correlate the amount of adjustment to the actual effect on at least one of the electrodes. As a result, such methods may also require iterative processes to dial in parallelism. Some of these methods are also vulnerable to shifting of the adjustment over time due to vibrations, such as shipping loads.